1. Field of the Invention
The present invention relates to a charged particle beam apparatus. More specifically, the present invention relates to a charged particle beam apparatus which irradiates a sample with a charged particle beam and detects charged particles emitted from an irradiation position.
2. Background Art
According to refinement of a pattern on a semiconductor wafer, it has been difficult to perform defect detection, observation, and length dimension measurement of a pattern width by an optical microscope type inspection device, and thus inspection, review, and length measurement based on an image-imaging have been performed by a scanning electron microscope (hereinafter, referred to as “Scanning Electron Microscopy (SEM)”). Hereinafter, an SEM will be described as an example.
An SEM irradiates a sample surface with an electron beam which converges on the sample surface, and detects secondary electrons or reflected electrons emitted from the irradiation position. Then, by two-dimensionally moving the irradiation position of the electron beam, a two-dimensional image is imaged.
An expected value of the number of secondary electrons and the number of reflected electrons which are emitted when the sample surface is irradiated with the electron beam is proportionate to a beam current controlling an electron beam emission quantity. However, a variation occurs in the number of secondary electrons or the number of reflected electrons which are emitted, and the variation is proportionate to the number of electrons which are emitted to the power of 0.5. The variation is a major cause of noise in an image detected by the SEM.
When an S/N ratio of an imaged image is considered, a signal component is proportionate to the beam current, and the variation which is a noise component is proportionate to the beam current to the power of 0.5, and thus the S/N ratio is proportionate to the beam current to the power of 0.5. According to these characteristics, it is necessary to image an image by using the largest beam current in order to obtain an image having an excellent S/N ratio. However, when the beam current increases, an aberration in an electron optical system increases, and thus a high resolution image is not able to be obtained. That is, a resolution and an S/N ratio are in a trade-off relationship.
In order to solve these problems, as a technique for improving an S/N ratio without using a beam current, a technique of reducing noise by frame addition is included. This is a method in which an image of the same region is imaged a plurality of times, each image is set as a frame, and a final image is synthesized by summing the frames. When this method is used, an S/N ratio is proportionate to the number of frames to the power of 0.5. However, an imaging time increases in proportion to the number of frames.
As described above, the S/N ratio of the imaged image is proportionate to a product of the beam current and the number of frames, to the power of 0.5 and the imaging time is proportionate to the number of frames.
When a defect of a semiconductor is reviewed or a dimension of a semiconductor pattern is measured by the SEM having these characteristics, in general, imaging is continuously performed in two types of conditions of low magnification and high magnification. For example, in a review SEM reviewing a defect of a semiconductor, the defect is displayed by being enlarged in high magnification based on defect coordinates output by an inspection device which detects the defect, but accuracy of the defect coordinates output by the inspection device is low, and the defect may not converge in a high magnification image. For this reason, before imaging a high magnification image, a defect image in low magnification and a reference image are compared to specify a defect position, and then an imaging region is adjusted and the high magnification image is imaged. Similarly, in a length measurement SEM, in order to determine a pattern to be measured, a low magnification image is imaged, and then an image is imaged in high magnification, and a line width or the like is measured from the imaged image with high accuracy.
As a method for improving a throughput of the continuous imaging, a method in which a beam current is switched to be a large beam current at the time of low magnification imaging, and the number of frames is decreased, and thus an imaging time at the time of the low magnification imaging is shortened is considered. This method is premised on the principle that both a resolution and an S/N ratio are obtained in high magnification imaging, and when the S/N ratio of the low magnification imaging is identical to that of the high magnification imaging, the resolution may be lower than that of the high magnification imaging.
Regarding the related art which realizes switching of a beam current, in JP-T-2010-519698, a plurality of opening diaphragms having different diameters is used in an opening diaphragm shielding a beam current, and the opening diaphragm is switched, and thus the beam current is switched. However, a switching rate of the beam current depends on an operating variation in the diameter of the opening diaphragm, or a half opening angle varies at the time of switching the beam current.
In JP-A-2009-26749, in order to switch a beam current, an electrostatic lens is superimposed on an electromagnetic lens, and a strength of a convergence effect of the electrostatic lens is changed, and thus an amount of beam shielded by an opening diaphragm is controlled. However, in order to realize an aberration in the electrostatic lens which is identical to that in the electromagnetic lens, a stable high voltage electric source is necessary, but a stable high voltage electric source which is able to perform rapid switching is not easily manufactured, and a cost increases as the number of components of the lens and the electric source increases.
In the related art for a beam current switching technique not using the above-described method of switching the opening diaphragm having different diameters, or the above-described method of switching the strength of the convergence effect of the electrostatic lens, JP-A-5-242845 is included. In JP-A-5-242845, an optical system is configured by three or more stages of electromagnetic lenses, and changes the strength of the convergence effect of the electromagnetic lens, and thus is able to switch at least any one of a beam current, a half opening angle, and a convergence position of a beam in a sample surface.
When lens action and an aberration occur to the same degree, the electromagnetic lens is able to be controlled with a small electric source compared to an electrostatic lens. In addition, the beam current is controlled by using the electromagnetic lens and an opening diaphragm, and the strength of the convergence effect of the electromagnetic lens, and thus the beam current is more easily and accurately adjusted to a desired beam current. In addition, when comparing to the methods of JP-T-2010-519698 and JP-A-2009-26749, it is not necessary to add an optical system for switching the beam current or the half opening angle, and thus an optical axis deviation due to a variation over time for positioning the optical system hardly occurs.
When the strength of the convergence effect of the electromagnetic lens is switched, a magnetic flux density in a trajectory through which a charged particle beam passes after being switched is changed over time by ringing of a lens current due to inductive reactance of the electromagnetic lens and a variation over time due to response delay. Further, a variation over time due to a magnetic aftereffect of a magnetic path material of the lens is superimposed on the variation over time in the magnetic flux density. Therefore, the strength of the convergence effect of the electromagnetic lens changes in a complex manner over time. Furthermore, ringing is a state in which a signal transitions to a convergence value while vibrating at the time of the strength of the signal changing, or an aspect thereof. For this reason, when the strength of the convergence effect of the electromagnetic lens is switched, the following problems occur.
First, since ringing occurs in the strength of the convergence effect of the electromagnetic lens, an amount of variation in the strength of the convergence effect of the electromagnetic lens per unit time is greater, and a standby time during which the variation converges to an amount which is allowable during imaging one image is prolonged.
Second, the standby time in which the variation in the strength of the convergence effect of the electromagnetic lens converges to the amount of variation which is allowable during imaging one image changes according to a switching width of the lens current, and thus it is difficult to ascertain a relationship between the switching width of the current and the standby time. For this reason, even when the switching width of the current is small, it is necessary to start the imaging after a standby time of a maximum switching width of the current or a time longer than the standby time has elapsed.
Third, a standby time in which the strength of the convergence effect of the electromagnetic lens converges in a normal state is prolonged.
As a result thereof, the strength of the convergence effect of the electromagnetic lens is changed, and thus in the charged particle beam apparatus having a configuration of switching at least any one of the beam current, the half opening angle, and the convergence position of the beam in the sample surface, it is not possible to perform the imaging by rapidly switching the beam current, the half opening angle, and the convergence position of the beam in the sample surface.